In recent years, the production of propylene oxide from propylene using hydrogen peroxide as an oxidant and a titanium-containing zeolite as a catalyst has been proposed. Methanol is a particularly preferred reaction solvent for such purposes, as it tends to promote high catalyst activity and selectivity. Epoxidation processes of this type are described, for example, in U.S. Pat. Nos. 5,591,875, 4,833,260, 5,621,122, 5,646,314, and 4,824,976, EP Pub. No. 0732327, and Clerici et al., J. Catalysis 129, 159-167 (1991), the teachings of which are incorporated herein by reference in their entirety. Although such processes are capable of providing exceptionally high selectivity to propylene oxide, minor quantities of certain by-products such as acetaldehyde are inevitably formed.
The methanol which is recovered following the separation of unreacted propylene and propylene oxide from the crude epoxidation reaction product often is contaminated with acetaldehyde. In addition to the acetaldehyde produced during epoxidation, acetaldehyde may be generated during the methanol recovery steps. Normally, it will be economically advantageous to recycle the recovered methanol for use in the epoxidation process. While low concentrations of acetaldehyde generally may be present in the epoxidation reaction mixture without deleterious effect on the epoxidation, in a continuous process the acetaldehyde will tend to accumulate in the methanol recycle stream to an unacceptable level. At high concentrations, for example, the accumulated acetaldehyde can contaminate the propylene oxide being produced. The complete separation of acetaldehyde from propylene oxide is difficult, however. It will thus be highly desirable to develop a means by which at least a portion of the acetaldehyde may be effectively separated from the recovered methanol prior to the methanol being reintroduced to the epoxidation reactor.